當subject (主語)是 he, she或 it 時, 會用has studied
e.g. He has studied for three hours.
She has studied for three hours.
而當 主語是 I , We, You, They 時, 會用 have studied
e.g.
I have studied for three hours.
We have studied....
They have studied.......
You have studied.....
至於 had studied 就用在 過去式 past tense 叫過去完成式, past perfect tense.
e.g. Yesterday I had studied English for two hours before I played computer games.